In electrophotography, an electrophotographic imaging member containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly, electrostatically charging its surface. The member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated area of the photoconductive insulating layer while leaving behind an electrostatic pattern in the non-illuminated area. A latent image may then result from either the charge-dissipated illuminated area or the charged, non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic member to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may exist in a number of forms. For example, the imaging member may be a homogeneous layer of a single material or may be a composite of more than one distinct layer. An example of a multilayered electrophotographic imaging member may comprise a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer and a charge transport layer. U.S. Pat. Nos. 4,265,990, 4,233,384 and 4,306,008 disclose examples of photosensitive members having at least two electrically operative layers, including a charge generating layer and a charge transport layer.
In multilayered imaging members, materials used for each layer preferably have desirable mechanical properties while also providing electrical properties necessary for the function of the device. If the material of one layer of the imaging device is changed in an attempt to improve a particular property, e.g., an electrical property, the change may have an adverse effect on mechanical properties such as delamination of one or more layers.
In a multi-layered electrophotographic imaging member having, inter alia, a charge generating layer and a charge transport layer, the photosensitivity of this electrophotographic imaging member depends on: 1) both the efficiency of conversion of absorbed photons into charge carriers (photogeneration efficiency of a charge generating material); and 2) the injection of those charges into the charge transport layer. If charge injection of the absorbed photons into the charge transport layer is limited, photosensitivity of the electrophotographic imaging member, measured by the rate of discharge upon exposure, will similarly be limited.
Other difficulties also exist in fabricating electrophotographic imaging members. In seamless imaging members, a conductive metal layer cannot be deposited in an economical manner. Similarly, vacuum coating techniques are expensive when coating seamless substrates. Thus, the use of conductive layers applied by other coating techniques becomes important.
Suitable and economical coating methods used for applying layers in multi-layer electrophotographic imaging members include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. These exemplary methods are known in the art. Solution coating is a preferred approach.
U.S. Pat. No. 4,082,551 to Steklenski et al. discloses a process of coating multiple layers onto an insulating, polyester substrate by applying solutions having the coating substance dissolved therein and drying each applied layer before coating a subsequent layer. In this case, the coated elements, when tested, indicate no chemical interaction between the photogenerating and conducting layers and essentially no change in the electrical resistivity of the conducting layer.
U.S. Pat. No. 4,571,371 to Yashiki discloses an electrophotographic photosensitive member having a charge generating layer and a charge transport layer. A dispersion of charge generating material dissolved in solvent was applied to a cured polyamide resin layer by soaking and dried at 100.degree. C. for 10 minutes to form a charge generating layer. Subsequently, a solution containing a charge transfer material was applied to the dried charge generating layer followed by drying at 100.degree. C. for 60 minutes to form a charge transfer layer.
U.S. Pat. No. 4,579,801 to Yashiki discloses a process for applying a dispersion of charge generating material in a solution containing a binder resin to a suitable substrate or dried underlayer. The charge generation layer can be formed by vapor deposition. The disclosure suggests that a charge transporting material, dissolved in a solution of resin, is applied using conventional methods to form a thin film.
U.S. Pat. No. 4,521,457 to Russell et al. discloses a process for simultaneously constraining two different coating materials, and forming on a substrate a continuous, unitary layer comprising adjacent "ribbons," each ribbon comprised of different materials and in edge-to-edge contact with an adjacent ribbon. The coated ribbons, thus applied, were dried in two zones, one at about 57.degree. C. and another at about 135.degree. C. Although the process is suitable for a number of applications, it is said to be particularly useful for producing electrophotographic imaging members utilizing multi-active layers.
Conventional electrophotographic imaging members, having at least a charge generating layer and a charge transport layer and made according to the above processes, suffer numerous disadvantages. For example, as discussed above, some electrophotographic imaging members suffer from poor charge acceptance and have limited photosensitivity due to limited injection of absorbed photons into the charge transport layer. In addition, charge transport materials may diffuse and come in contact with the conductive layer, adversely affecting the electrophotographic imaging member. Notably, devices manufactured using conventional processes have limited photoresponse.